In recent years, in the automotive field, etc., improvement in fuel efficiency and motion performance have been sought, while improvement in collision safety has also been sought. As a chassis of a motorcar, in order to balance reduced weight and high stiffness, a chassis structure referred to as a “monocoque body,” wherein a frame and a body are integrally formed, is generally used. Further, in the chassis of a motorcar, in order to maintain a survival space for a driver and a passenger in a collision, a shock absorbing structure is generally used, wherein a space (for example, an engine room or a luggage room) other than a cabin is preferentially collapsed, so that impact load applied to the cabin is attenuated as much as possible and the deformation of the cabin is minimized.
Therefore, in order to constitute a chassis structure with high collision-safety performance, it is important to effectively absorb the impact energy at the time of collision. To this end, a shock absorbing member for effectively absorbing the impact energy at the time of collision has been strenuously developed (for example, see Patent Literatures 1 to 15).
Generally, as a shock absorbing member, a hollow columnar thin-walled structure (or a hollow columnar member) manufactured by jointing press-formed steel plates by welding, etc., is used. In order to balance reduced weight and high stiffness, as described above, the shock absorbing member is constituted by a hollow member having a polygonal cross-section such as a tetragon or a hexagon. Such a shock absorbing member is used as a front-side member of a chassis, for example, and absorbs the impact energy by buckling in an axial direction (or axial crushing) when impact load is applied to one end of the member at the time of collision. Therefore, in order to improve the shock-absorbing performance, it is important to effectively generate such buckling and increase the buckling load thereof.
In the prior art, in order to solve the above problems in terms of material, a relatively thick or relatively high-strength steel plate is used to manufacture a shock absorbing plate so as to increase the buckling load. On the other hand, as a structural measure, a dimple (or a bead) providing the origin of the buckling is arranged so as to effectively generate the buckling. Further, by forming a cross-section of the hollow shock absorbing member as a polygonal shape, the buckling load is increased.
However, when the plate thickness of the above shock absorbing member is increased, the weight of the member is increased, whereby the weight of a chassis including the shock absorbing member is also increased. As a result, fuel efficiency and driving performance of a motorcar are deteriorated. Further, in a high-strength steel plate, the degree of elongation is generally decreased in inverse proportion to the strength thereof. Therefore, the formability of a high-strength steel plate is not good, and thus there are limitations to increasing the strength of a steel plate for the shock absorbing member at present.
Incidentally, when the buckling load of the shock absorbing member is solely increased, a minimum impact load for generating the buckling is increased. In this case, the impact load applied to the shock absorbing member is not absorbed by the deformation of the shock absorbing member, whereby the impact load with no change is transmitted to another structure such as a cabin. Further, a risk of injury to driver and a passenger is increased, since a portion which is not to be deformed is buckled; it is difficult to keep a survival space for the drive and the passenger due to the deformation of the cabin; or a significant change of acceleration is applied to the driver and the passenger.
Therefore, for example, the shock absorbing member is configured as a straight member in order to keep the cross-section from a start end of the buckling constant as much as possible, and secure a certain amount of deformation due to the buckling. Further, in order to reduce an initial impact load applied to the shock absorbing member, the shock absorbing member is stably buckled into concertinas due to an arrangement of the beads as described above.
However, there is no firm theory for determining the above arrangement of the beads, and at present, the arrangement is determined by repeatedly buckling test or a computer simulation regarding the shock absorbing member. Accordingly, it is necessary to repeatedly carry out the above test or simulation in relation to each kind of chassis, whereby design efficiency is deteriorated. Moreover, since various load conditions or buckling modes predicted when actual collision cannot be dealt with, it is very difficult to optimize the arrangement of the beads by using the above techniques.